JavaScript prototype to find twin primes

When 2 prime numbers are subtracted and their result is 2 then they are known as twin primes.

Here is the JavaScript prototype to find twin primes up to a given limit. Here is the code snippet.

Code (Run Code):

arr = [];

function checkPrime(num) {

if (num == 1) {

return false;

} else if (num == 2) {

return true;

}

for (var x = 2; x < num; x++) {

if (num % x == 0) {

return false;

}

}

return true;

}

function primeNumbers() {

num = Number(document.getElementById('number').value);

for (var j = 1; j < num; j++) {

if (checkPrime(j)) {

arr.push(j);

}

}

}

function twinPrimes() {

var text = '';

arr = [];

primeNumbers();

text += 'Prime Numbers:';

text += '<br>' + arr;

text += '<br> Twin Primes:'

for (var i = 0; i < arr.length; i++) {

for (var x = i + 1; x < arr.length; x++) {

if ((arr[i] - arr[x]) == -2) {

text += '<br>' + arr[x] + ' and ' + arr[i];

}

}

}

document.getElementById('result').innerHTML = text;

}